The present invention relates to a method of casting metal objects in moulds and to an arrangement for carrying out the method. The method and arrangement are of the kind (a) which provides a mould having at least one casting cavity with a filling duct opening into the lowermost part thereof; (b) which provides a supply of molten metal to the cavity with a sufficient pressure to be able to rise at least to the uppermost part of the casting cavity; and (c) which connects the supply to the filling duct allowing the molten metal to fill the casting cavity or cavities by ascending to the uppermost limit(s) thereof.
In recent years, manufacturers of cast articles have expressed a desire to be able to cast articles of aluminium or its alloys using a more adaptable technology than that available with conventional die-casting techniques. As it is widely known that the use of disposable sand moulds allows for far greater flexibility and adaptability than conventional die casting, this desire has focussed the attention on the use of sand moulds for casting aluminium objects. This has already been done for some time, but generally only for casting objects having relatively large cross-section, as attempts to cast thin-walled objects have in most cases proved unsuccessful.
It was found at relatively early stage that it would in any case be necessary to use casting cavities of the type being filled from below, as filling from the top invariably caused undesirable splashing leading to oxidation and formation of highly unpleasant slag in the form of xe2x80x9cbeardxe2x80x9d, and there could even be a risk of explosion caused by a chemical reaction between the aluminium and moisture in the mould, liberating hydrogen that could form an explosive mixture with any air present.
Further, when thus having decided to use moulds with casting cavities to be filled from below, it was found that the rate, at which the molten metal enters the casting cavities, was a highly critical parameter. Thus, a too low rate could not ensure that the mould was filled xe2x80x9cin timexe2x80x9d, i.e. before the metal began to solidify in the casting cavities, whereas a too high filling rate could cause turbulence, facilitating oxidation and entailing a risk of mechanical damage to the walls of the casting cavities.
It is the object of the present invention to provide a method of the kind referred to initially, with which it is possible to avoid the difficulties explained above and to make it possible to use disposable sand moulds for producing problematic castings, such as thin-walled articles of aluminum or its alloys. This object is achieved by, according to the invention, in that the connecting and filling step comprises the steps of (a) connecting the supply and the filling duct to an external riser extending upwardly to a higher level than the uppermost limit of the casting cavity or cavities and capable of temporarily containing a quantity of the molten metal, and (b) allowing the molten metal in the external riser to ascend toward the higher level at least until the casting cavity or cavities is/are completely filled with molten metal. By so doing, it is possible to achieve close control of the rate, at which the casting cavities are filled with molten metal, as the external riser acts as a xe2x80x9cpressure bufferxe2x80x9d preventing both too high and too low filling rates.
The present invention also relates to an arrangement for carrying out the method of the invention. This arrangement includes (a) a supporting means for temporarily supporting a mould having at least one casting cavity, the lowermost part of each such cavity communicating with a filling duct, at least one end of which is open to the outside of the mould, b) a mould-filling means adapted to contain molten metal and to transfer the latter under a controlled pressure to the filling duct, being temporarily connectable to the latter through a coupling means adapted to engage the filling-duct open end in a liquid-tight manner, and c) an external riser means communicating with the coupling means and in the filling position of the mould extending upwardly to a higher level than the uppermost limits of the casting cavity or cavities. As will be explained in the following detailed part of the present description, such an apparatus is suitable for carrying out the method.
Further advantageous embodiments of the method and the arrangement, the effects of which will be evident from the following detailed part of the present description, are set forth hereinafter.